U.S. patent application number 11/362768 was filed with the patent office on 2006-08-31 for electroluminescent phosphor, method for producing the same and device containing the same.
This patent application is currently assigned to FUJI PHOTO FILM CO., LTD. Invention is credited to Kyohei Ogawa.
Application Number | 20060192486 11/362768 |
Document ID | / |
Family ID | 36931413 |
Filed Date | 2006-08-31 |
United States Patent
Application |
20060192486 |
Kind Code |
A1 |
Ogawa; Kyohei |
August 31, 2006 |
Electroluminescent phosphor, method for producing the same and
device containing the same
Abstract
An electroluminescent phosphor comprising: ZnS-based phosphor
particles and a coating layer provided on a surface of the
particle, wherein the particles have an average particle size of
from 0.1 to 20 .mu.m, and a coefficient of variation in a particle
size distribution of less than 35%, and a content of particles
having 10 or more stacking faults with an interplanar spacing of 5
nm or less is 30% or more based on all of the ZnS-based phosphor
particles.
Inventors: |
Ogawa; Kyohei;
(Minami-Ahigara-shi, JP) |
Correspondence
Address: |
SUGHRUE MION, PLLC
2100 PENNSYLVANIA AVENUE, N.W.
SUITE 800
WASHINGTON
DC
20037
US
|
Assignee: |
FUJI PHOTO FILM CO., LTD
|
Family ID: |
36931413 |
Appl. No.: |
11/362768 |
Filed: |
February 28, 2006 |
Current U.S.
Class: |
313/509 |
Current CPC
Class: |
H05B 33/14 20130101;
C09K 11/584 20130101 |
Class at
Publication: |
313/509 |
International
Class: |
H05B 33/00 20060101
H05B033/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 28, 2005 |
JP |
P. 2005-053415 |
Claims
1. An electroluminescent phosphor comprising: ZnS-based phosphor
particles and a coating layer provided on a surface of the
particle, wherein the particles have an average particle size of
from 0.1 to 20 .mu.m, and a coefficient of variation in a particle
size distribution of less than 35%, and a content of particles
having 10 or more stacking faults with an interplanar spacing of 5
nm or less is 30% or more based on all of the ZnS-based phosphor
particles.
2. The electroluminescent phosphor as claimed in claim 1, wherein
the average particle size of the particles is from 15 to 20
.mu.m.
3. The electroluminescent phosphor as claimed in claim 1, wherein a
ratio of an average thickness of the coating layer to the average
particle size of the particles is from 0.001 to 0.1.
4. The electroluminescent phosphor as claimed in claim 1, wherein
the ZnS-based phosphor particles contain at least one element
selected from the group consisting of Cu, Mn, Ag and rare earth
elements.
5. The electroluminescent phosphor as claimed in claim 1, wherein
the ZnS-based phosphor particles contain at least one element
selected from the group consisting of Cl, Br, I and Al.
6. The electroluminescent phosphor as claimed in claim 1, wherein
the ZnS-based phosphor particles contain at least one element
selected from the group consisting of Au, Sb, Bi, Cs and Pt.
7. The electroluminescent phosphor as claimed in claim 1, wherein
the ZnS-based phosphor particles contain 1.times.10.sup.-7 to
5.times.10.sup.-4 mol of Au per mol of ZnS.
8. The electroluminescent phosphor as claimed in claim 1, wherein
the ZnS-based phosphor particles contain 1.times.10.sup.-7 to
1.times.10.sup.-3 mol of Pt per mol of ZnS.
9. The electroluminescent phosphor as claimed in claim 1, wherein
the coating layer contains at least one compound selected from the
group consisting of oxides, nitrides, hydroxides, fluorides,
phosphates, diamond carbon and organic compounds.
10. A method for producing an electroluminescent phosphor as
claimed in claim 1, which comprises while fluidizing the ZnS-based
phosphor particles in a presence of a fluidization promoter having
a larger average particle size than the average particle size of
the particles, supplying a material for making the coating layer
thereto and piling up the material on a surface of the particles or
reacting the material with the particles so as to form the coating
layer.
11. A dispersion type electroluminescent device comprising: an
opposing pair of electrodes at least one of which is transparent; a
phosphor layer provided between the electrodes; and a dielectric
layer provided between the electrodes, wherein the phosphor layer
contains the electroluminescent phosphor as claimed in claim 1.
12. The dispersion type electroluminescent device as claimed in
claim 11, wherein the phosphor layer has a thickness of from 40 to
100 .mu.m.
13. The dispersion type electroluminescent device as claimed in
claim 11, further comprising an intermediate layer provided between
the transparent electrode and the phosphor layer.
14. The dispersion type electroluminescent device as claimed in
claim 13, wherein the intermediate layer contains at least one of
an organic polymer compound and an inorganic compound, and the
intermediate layer has a thickness of from 10 nm to 100 .mu.m.
Description
FIELD OF THE INVENTION
[0001] This invention relates to a ZnS-based electroluminescent
(hereinafter sometimes referred to as "EL") phosphor, a method of
producing the same and an EL device containing the same.
BACKGROUND OF THE INVENTION
[0002] EL devices are roughly divided into dispersion type EL
devices wherein phosphor particles are dispersed in a dispersant
and thin film EL devices wherein a thin phosphor film is inserted
between dielectric layers. Dispersion type EL devices have the
following characteristics, i.e., they are available in a
constitution made up of flexible materials with the use of a
plastic board without resorting to a high-temperature process, they
can be produced by a relatively simple process at a low cost
without using vacuum apparatus, the color of emitting light can be
easily controlled by mixing phosphor particles of multiple types
emitting lights of different colors, a relatively large area can be
easily obtained and so on. Owing to these characteristics, attempts
have been made to develop dispersion type EL devices as flat light
sources. With the recent diversification in electronic devices,
dispersion type EL devices are frequently employed as display
materials for decorative purposes as well as image display
devices.
[0003] It is known that EL devices suffer from the problem of
lowering in brightness caused by moisture in EL phosphors. To
prevent an EL phosphor from worsening, it has been a practice to
form a coating layer made of a moisture-proof inorganic material on
the surface of the EL phosphor as described in Japanese Patent No.
3187481, JP-A-11-204254, U.S. Pat. No. 5,643,496, JP-A-11-260557,
JP-A-2002-226845, Japanese Patent No. 3286264 and JP-A-2002-124391.
In the methods described in Japanese Patent No. 3187481 and
JP-A-11-204254, however, there arises the problem that the luminous
efficiency is lowered due to oxygen, steam, heat and so on in the
step of forming a coating layer. On the other hand, U.S. Pat. No.
5,643,496, JP-A-11-260557, JP-A-2002-226845, Japanese Patent No.
3286264 and JP-A-2002-124391 indicate that the luminous efficiency
is improved by coating. However, these methods are still
insufficient for achieving both of a high brightness and a high
luminous efficiency. Thus, it is impossible to achieve a high
brightness and a high luminous efficiency by using these known
techniques. Namely, these techniques are still insufficient for
establishing a high brightness and such a high luminous efficiency
as being available in practice.
SUMMARY OF THE INVENTION
[0004] Accordingly, an object of the invention is to provide an EL
phosphor achieving both of a high luminous efficiency and a high
brightness and having a long life and a method of producing the
same to thereby obtain an excellent EL device.
[0005] As the results of intensive studies, we have found out that
both of the highest luminous efficiency and brightness can be
achieved at high level by appropriately selecting the particle size
(in particular, selecting a particle size range of from 15 to 20
.mu.m), in the case of coated particles wherein gold and platinum
are added and a large amount of copper is added as an activator to
phosphor particles having a small coefficient of variation in the
particle size distribution and a high stacking fault ratio. We have
further found out that in an EL device containing these phosphor
particles, both of the highest luminous efficiency and brightness
can be achieved at high level by selecting a phosphor layer
thickness range of from 40 .mu.m to 100 .mu.m.
[0006] Accordingly, the invention is as follows.
[0007] (1) An electroluminescent phosphor containing at least
ZnS-based phosphor particles and a coating layer formed on the
surface thereof, wherein the average particle size of the particles
is from 0.1 to 20 .mu.m, the coefficient of variation in the
particle size distribution thereof is less than 35% and the content
of particles having 10 or more stacking faults with interplanar
spacings of 5 nm or less is 30% or more based on all particles.
[0008] (2) An electroluminescent phosphor as described in the above
(1) wherein the average particle size of the particles is from 15
to 20 .mu.m.
[0009] (3) An electroluminescent phosphor as described in the above
(1) or (2) wherein the ratio of the average thickness of the
coating layer to the average particle size of the particles is from
0.001 to 0.1.
[0010] (4) An electroluminescent phosphor as described in any one
of the above (1) to (3) wherein the ZnS-based phosphor particles
contain as an activator at least one element selected from the
group consisting of Cu, Mn, Ag and rare earth elements.
[0011] (5) An electroluminescent phosphor as described in any one
of the above (1) to (4) wherein the ZnS-based phosphor particles
contain as a coactivator at least one element selected from the
group consisting of Cl, Br, I and Al.
[0012] (6) An electroluminescent phosphor as described in any one
of the above (1) to (5) wherein the ZnS-based phosphor particles
contain as an additive at least one element selected from the group
consisting of Au, Sb, Bi, Cs and Pt.
[0013] (7) An electroluminescent phosphor as described in any one
of the above (1) to (6) wherein the ZnS-based phosphor particles
contain 1.times.10.sup.-7 to 5.times.10.sup.-4 mol of Au per mol of
ZnS.
[0014] (8) An electroluminescent phosphor as described in any one
of the above (1) to (7) wherein the ZnS-based phosphor particles
contain 1.times.10.sup.-7 to 1.times.10.sup.-3 mol of Pt per mol of
ZnS.
[0015] (9) An electroluminescent phosphor as described in any one
of the above (1) to (8) wherein the coating layer contains at least
one compound selected from the group consisting of oxides,
nitrides, hydroxides, fluorides, phosphates, diamond carbon and
organic compounds.
[0016] (10) A method of producing an electroluminescent phosphor as
described in any one of the above (1) to (9) which comprises while
fluidizing the ZnS-based phosphor particles in the presence of a
fluidization promoter having a larger average particle size than
the average particle size of the particles, supplying the material
of the coating layer thereto and thus piling up the material on the
surface of the particles or reacting the material with the
particles to thereby form the coating layer.
[0017] (11) A dispersion type electroluminescent device comprising
a phosphor layer held between a pair of electrodes facing each
other, at least one of which is transparent, and a dielectric layer
wherein the phosphor layer contains an electroluminescent phosphor
as described in any one of the above (1) to (9).
[0018] (12) A dispersion type electroluminescent device as
described in the above (11) wherein the thickness of the phosphor
layer is from 40 to 100 .mu.m.
[0019] (13) A dispersion type electroluminescent device as
described in the above (11) or (12) wherein at least one
intermediate layer is provided between the transparent electrode
and the phosphor layer.
[0020] (14) A dispersion type electroluminescent device as
described in any one of the above (11) to (13) wherein the
intermediate layer is made of an organic polymer compound, an
inorganic compound or a complex thereof and the thickness of the
intermediate layer is from 10 nm to 100 .mu.m.
[0021] As the results of extensive studies, the inventors have
found out that specific surface-coated phosphor particles, i.e.,
phosphor particles having a particle size falling within definite
range (in particular, from 15 to 20 .mu.m) and a small coefficient
of variation thereof and having an inner structure with a large
number of planar stacking faults exhibit extremely improved effects
of achieving a high luminous efficiency, a high brightness and a
long life. Moreover, it is found out that the effects of achieving
a high luminous efficiency, a high brightness and a long life can
be much improved by adding gold and by adding platinum.
[0022] Furthermore, it is found out that the effects of achieving a
high luminous efficiency, a high brightness and a long life can be
much improved by applying the phosphor particles as described above
to an EL device having a phosphor layer thickness of from 40 to 100
.mu.m. In particular, it is found out that the addition of gold and
the addition of platinum bring about an increase in the luminous
efficiency of coated phosphor particles having a particle size of
from 15 to 20 .mu.m. Accordingly, appropriate combination of these
techniques highly efficiently contributes to an increase in the
luminous efficiency and extension of the life as a whole.
[0023] In the invention, use is made of phosphor particles with a
specific structure, i.e., phosphor particles having a small
particle size and a small coefficient of variation and having an
inner structure with a large number of planar stacking faults.
Thus, these particles give EL light emission at an extremely high
efficiency and a high brightness, though they have a coating layer.
In the case of employed as an EL device, moreover, these phosphor
particles can highly improve the durability of the EL device owing
to the coating layer. This is seemingly because the deterioration
of the phosphor and ion elution from the phosphor particles can be
effectively prevented by the formation of the coating layer.
[0024] The EL phosphor of the invention, which has a small particle
size and a narrow particle size distribution, shows a favorable
dispersibility and can form an even phosphor layer. Thus, the
coarseness (granularity) of the light emission can be remarkably
improved, which makes it highly suitable for transmission
photographs of high image qualities and inkjet transmission
lighting.
[0025] As a method particularly suitable for the formation of a
coating layer on such a small-sized EL phosphor, it is furthermore
found out according to the invention that the coating can be highly
efficiently formed at a high reproducibility without causing
aggregation of the particles by fluidizing the EL phosphor under
specific conditions with the use of a fluidization promoter.
[0026] In an EL device containing the EL phosphor of the invention,
effects of preventing a transparent electrode from deterioration
and further improving the durability of the EL device can be
achieved by providing an intermediate layer between the transparent
electrode and the phosphor layer. Even in the case of using an EL
phosphor having a coating layer with poor ion-barrier properties,
the effect of preventing the transparent electrode from
deterioration can be obtained thereby.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] FIG. 1 is a drawing schematically showing a fluidized bed
reactor for producing the coated EL phosphor particles of the
invention.
[0028] FIG. 2 is a drawing schematically showing an agitated bed
reactor for producing the coated EL phosphor particles of the
invention.
[0029] FIG. 3 is a drawing schematically showing a vibrated bed
reactor for producing the coated EL phosphor particles of the
invention.
[0030] FIG. 4 is a drawing schematically showing a rotated bed
reactor for producing the coated EL phosphor particles of the
invention.
[0031] FIG. 5 is a drawing schematically showing a liquid phase
reactor for producing the coated EL phosphor particles of the
invention.
[0032] FIG. 6 is a drawing schematically showing a compound
particle-constructing apparatus for producing the coated EL
phosphor particles of the invention.
[0033] FIG. 7 is a drawing schematically showing the cross section
of the EL device of the invention.
DESCRIPTION OF THE REFERENCE NUMERALS
[0034] TABLE-US-00001 52 intermediate layer 53 phosphor layer 54
dielectric layer 55 transparent electrode layer 56 PET support 57
back electrode layer 58 moisture-proof film
DETAILED DESCRIPTION OF THE INVENTION
[EL Phosphor]
(ZnS-Based EL Phosphor Core Particles)
[0035] The ZnS-based phosphor particles to be used in the EL
phosphor of the invention have an average particle size of from 0.1
to 20 .mu.m, preferably from 15 to 20 .mu.m. The coefficient of
variation in the particle size distribution thereof is less than
35%, preferably less than 30%. Owing to these characteristics, the
dispersibility of the EL phosphor particles and the filling rate of
the EL phosphor particles in the phosphor layer can be elevated and
the coarseness (granularity) of the light emission of an EL device
can be improved.
[0036] Such EL phosphor core particles can be obtained by, for
example, the following method.
[0037] As a precursor from which the ZnS-based phosphor particles
are obtained, use may be made of marketed ZnS of a high purity.
However, it is preferred to employ a precursor to which an
activator has been uniformly added. To obtain such a precursor, use
may be preferably made of the hydrothermal synthesis method, the
homogeneous precipitation method or the spray thermal decomposition
method. By any of these methods, a Zn salt and an activator salt,
which are both dissolved in a solvent, are reacted together to form
ZnS so as to give a precursor wherein the activator is incorporated
into the ZnS. As the activator, it is preferable to use Cu, Mn, Ag
or a rare earth element. It is more preferable to use Cu. The
addition level of the activator varies depending on the activator
type. For example, Cu may be added preferably in an amount of from
1.times.10.sup.-4 to 1.times.10.sup.-2 mol per mol of ZnS, still
preferably from 5.times.10.sup.-4 to 5.times.10.sup.-3 mol. In the
case of using a precursor free from an activator, ZnS is dispersed
in water and a water-soluble Cu compound (for example, CuSO.sub.4,
Cu(NO.sub.3).sub.2 or the like) is added to the obtained
suspension. Thus a precursor in which Cu.sub.xS has been deposited
on the surface of ZnS particles is prepared. In this case, it is
preferable to wash the suspension with distilled water several
times after the completion of the reaction to thereby remove
ZnSO.sub.4 formed as a by-product.
[0038] As a coactivator, use can be made of Cl, Br, I and Al. It is
preferable that the addition level of the coactivator is equivalent
to that of the activator. Such a coactivator is introduced from a
flux as will be described hereinafter. In the case of Al, however,
it should be separately added in the form of, for example,
Al(NO.sub.3).sub.3.
[0039] In addition to the activator and the coactivator, it is
preferable to add Au, Sb, Bi, Cs or Pt as an additive and the
addition of Au is particularly preferred. It is assumed that the
addition of Au prevents Cu.sub.xS crystals, from which electrons of
the EL phosphor are generated, from deterioration. Thus, it is
found out that the Au thus added effectively contributes to the
improvement in the luminous efficiency and the extension of the
life, together with the means of increasing the luminous
efficiency, elevating the brightness and prolonging the life
according to the invention. Particularly remarkable effects are
observed in an EL phosphor which contains a large amount of Cu as
the activator. The addition level of Au is preferably from
1.times.10.sup.-7 to 5.times.10.sup.-4 mol per mol of ZnS, still
preferably from 5.times.10.sup.-7 to 1.times.10.sup.-4 mol.
[0040] By adding Pt, the luminous efficiency and the brightness can
be furthermore elevated. It is preferable to add Pt to zinc sulfide
in an amount of from 1.times.10.sup.-7 mol to 1.times.10.sup.-3 mol
per mol of zinc sulfide, still preferably from 1.times.10.sup.-6
mol to 5.times.10.sup.-4 mol.
[0041] It is preferable that these metals are added together with a
zinc sulfide powder and a definite amount of copper sulfate to
deionized water, thoroughly mixed in the form of a slurry, then
dried and baked together with a flux to thereby incorporate the
metals into zinc sulfide particles. Alternatively, it is preferred
that a powdery complex containing these metals is mixed with a flux
and then baked with the use of the flux to thereby incorporate the
metals into the zinc sulfide particles. In each case, an arbitrary
compound containing the metal to be used can be employed as a
starting compound for adding the metal. However, it is more
preferable to employ a complex in which oxygen or nitrogen is
coordinated with the metal or metal ion. As a ligand, either an
inorganic compound or an organic compound may be used.
[0042] The EL phosphor may be baked by a solid phase reaction
similar to the conventional methods. First, the precursor
containing the activator is mixed with a flux also serving as a
halogen coactivator source such as an alkali metal halide, an
alkaline earth metal halide, an ammonium halide or zinc halide or
an Al compound in the case of using Al as the coactivator. In the
case of adding Cs as an additive, a Cs halide is further added and
mixed. The mixing may be performed by dry-mixing in a mortar, a
tumbler mixer or the like. Alternatively, distilled water may be
once added to the mixture to give a suspension which is then dried
to thereby give a more homogeneous mixture. The flux is added
preferably in an amount of from 1 to 80% by mass based on ZnS,
still preferably from 20 to 60% by mass. In the case where the flux
is used in an excessively small amount, the crystal development
cannot normally proceed in some cases. On the other hand, use of
the flux in an excessively large amount results in the generation
of a corrosive toxic gas. The mixture is packed in an alumina
crucible and baked at a baking temperature of from 900 to
1200.degree. C. To allow sufficient crystal development and even
distribution of the activator in ZnS, the baking time preferably
ranges from 30 minutes to 12 hours, still preferably from 1 to 6
hours. As the baking atmosphere, use can be made of an oxidative
atmosphere such as air or oxygen, an inert atmosphere such as
nitrogen or argon, a reductive atmosphere such as a
hydrogen-nitrogen mixture or a carbon-oxygen mixture, or a
sulfidizing atmosphere such as hydrogen sulfide or carbon
disulfide.
[0043] After taking out the baked mixture from the crucible, it is
preferable to sufficiently and repeatedly wash the baked mixture
with an acid and water to thereby remove the excessive flux,
by-products of the reaction, ZnO formed by the oxidization of ZnS
and so on. The washed particles are then dried by using a vacuum
dryer or the like so as to give an intermediate phosphor having a
wurtzite crystal system.
[0044] It is preferable that a stress is applied to the baked
intermediate phosphor and then re-baked to thereby increase the
stacking fault density and elevate the brightness. To apply a
stress on the phosphor particles, use may be made of a ball mill,
ultrasonic wave, hydrostatic pressure or the like. It is preferable
in any case to uniformly apply a load to the particles at such an
extent as not breaking the phosphor particles. After applying the
stress, the phosphor is re-baked at a temperature of from 500 to
900.degree. C. In this step, it is favorable to add a compound of
Sb or Bi, if needed, so as to prolong the life of the EL phosphor.
The addition level thereof preferably ranges from 1.times.10.sup.-5
to 1.times.10.sup.-3 per mol of ZnS. Thus, most of the crystals are
converted into the sphalerite structure. The re-baking may be
carried out under the same conditions (baking time, atmosphere,
etc.) as those employed in the baking.
[0045] By controlling the baking conditions and the like as
described above, it is possible to obtain a ZnS-based EL phosphor
having an average particle size of the from 0.1 to 20 .mu.m, having
a coefficient of variation in the particle size distribution of
less than 35% and containing particles having 10 or more stacking
faults with interplanar spacings of 5 nm or less in an amount of
30% or more based on all particles. However, the method of
producing an ZnS-based EL phosphor is not restricted thereto. For
example, an EL phosphor having a desired average particle size or
particle size distribution can be obtained by preparing an EL
phosphor having an average particle size exceeding 15 .mu.m and
then classifying the particles with the use of a dry sieve, a wet
sieve, a gas cyclone, a liquid cyclone, a hydraulic elutriation or
the like. It is also possible to obtain an EL phosphor having a
desired average particle size or particle size distribution by
preparing an EL phosphor having an average particle size exceeding
15 .mu.m and then grinding the particles by using a mortar, a ball
mill, a jet mill or the like.
(Formation of Coating Layer)
[0046] The EL phosphor of the invention is obtained by forming a
coating layer on the surface of the core particles of the EL
phosphor. The average thickness of the coating layer is preferably
from 0.01 to 1 .mu.m, still preferably from 0.05 to 0.5 .mu.m. The
term "average thickness of the coating layer" means the value
determined by selecting at least ten particles from the
cross-sectional SEM image of EL phosphor particles having the
coating layer formed thereon, measuring the thickness of the
coating layer at arbitrary three points per particles and
calculating the mean.
[0047] So long as the average thickness of the coating layer falls
within the range as defined above, a favorable moisture-proofness
and ion-barrier properties can be obtained. It is also favorable
that a lowering in the brightness and an increase in the threshold
voltage for light emission are scarcely induced thereby without
decreasing the electrical field intensity of the EL phosphor
particles.
[0048] It is also preferable that the coating layer has a thickness
suitable for the average particle size. In the case of forming a
coating layer of 1 .mu.m on particles of 1 .mu.m in size, for
example, a lowering in the electrical field intensity of the
particles is scarcely induced. That is to say, the ratio of the
average thickness of the coating layer to the average particle size
of the particles is preferably from 0.001 to 0.1, still preferably
from 0.002 to 0.05.
[0049] Although the composition of the coating layer is not
particularly restricted, it is possible to employ an oxide, a
nitride, a hydroxide, a fluoride, a phosphate, diamond carbon or an
organic compound therefor. It is also preferable to employ a
mixture thereof, mixed crystals, a multilayer film and so on. More
specifically speaking, use can be preferably made of SiO.sub.2,
Al.sub.2O.sub.3, TiO.sub.2, ZrO.sub.2, HfO.sub.2, Ta.sub.2O.sub.5,
Y.sub.2O.sub.3, La.sub.2O.sub.3, CeO.sub.2, BaTiO.sub.3,
SrTiO.sub.3, PZT, Si.sub.3N.sub.4, AlN, Al (OH).sub.3, MgF.sub.2,
CaF.sub.2, Mg.sub.3(PO.sub.4).sub.2, Ca.sub.3(PO.sub.4).sub.2,
Sr.sub.3(PO.sub.4).sub.2, Ba.sub.3(PO.sub.4).sub.2, a fluororesin
and so on. It is also preferable that the coating layer is free
from pinholes or cracks and has a continuous structure.
[0050] The coating layer of the invention can be formed by, for
example, the following method.
[0051] As a first method of forming a coating layer, a method
comprising while fluidizing EL phosphor core particles, supplying
the material of the coating layer thereto and thus piling up the
material on the surface of the particles or reacting the material
with the particles to thereby form the coating layer may be
cited.
[0052] The EL phosphor core particles can be fluidized by using an
appropriate known method. For example, it is possible to employ a
method with the use of fluidized bed, an agitated bed, a vibrated
bed or a rotated bed. In a fluidized bed, EL phosphor core
particles are packed into a cylindrical container and then the
suspended and fluidized by supplying a carrier gas from the bottom
of the container via a porous plate, as shown in, for example, FIG.
1. In an agitated bed, EL phosphor core particles are packed and
directly fluidized by using an impellar agitator or the like, as
shown in, for example, FIG. 2. In a vibrated bed, EL phosphor core
particles packed in a container are mechanically or electrically
vibrated together with the container, as shown in, for example,
FIG. 3. In a rotated bed, EL phosphor core particles are packed in
a cylindrical container located horizontally or inclinedly and then
rotating the container to thereby fluidize the particles, as shown
in, for example, FIG. 4.
[0053] To obtain an even coating layer, it is particularly
preferred to employ a fluidized bed. With a decrease in the
particle size, the EL phosphor tends to aggregate and thus the
fluidization becomes difficult. Therefore, it is favorable to add a
fluidization promoter having a larger average particle size than
the average particle size of the EL phosphor core particles. It is
preferable that the fluidization promoter has a particle size 2 to
5 times larger than the average particle size of the EL phosphor
core particles. As the fluidization promoter, it is preferable to
employ a substance which is inert to the EL phosphor at the
reaction temperature, for example, SiO.sub.2, Al.sub.2O.sub.3,
ZrO.sub.2 or the like. It is also preferable that the fluidization
promoter has a spherical shape whereby the highest fluidity can be
established.
[0054] It is preferable to finally remove coarse particles or
aggregated particles by using, for example, a dry sieve. Thus, it
is possible to obtain a ZnS-based EL phosphor having an average
particle size of from 0.1 to 20 .mu.m, a coefficient of variation
in the particle size distribution of less than 35% and contains
particles having 10 or more stacking faults with interplanar
spacings of 5 nm or less in an amount of 30% or more based on all
particles.
[0055] By using the above method, a coating layer made of an oxide,
a nitride, a hydroxide, diamond carbon, etc. can be formed. For
example, a TiCl.sub.4 solution is vaporized by bubbling N.sub.2 gas
thereinto and then reacted with the N.sub.2 gas containing steam on
the surface of EL phosphor core particles to form a TiO.sub.2
precursor coating. An AlN coating can be formed by reacting an
alkyl aluminum with anhydrous ammonia gas.
[0056] As a second method of forming a coating layer, a method
comprising while dispersing EL phosphor core particles in a
solvent, supplying the material of the coating layer thereto and
thus piling up the material on the surface of the particles or
reacting the material with the particles to thereby form the
coating layer may be cited.
[0057] In this method, EL phosphor core particles can be introduced
together with the solvent into a reactor and then dispersed by
using an impellar agitator or the like. It is preferable that the
reactor has a cylindrical shape with a conical or semispherical
bottom. Agitating blades may be screw blades, twisted blades,
paddles or the like. It is more preferable to employ screw-paddle
blades whereby agitating streams in the circumferential direction
of the agitation axis as well as in the perpendicular direction. As
FIG. 5 shows, it is preferable to provide a strainer around the
agitating blades so as to form a stronger agitation stream in the
perpendicular direction. As the solvent, use can be preferably made
of water, an organic solvent or a mixture thereof. As a specific
solvent, it is also possible to use urea having been molted by
heating to the melting temperature or above. Furthermore, it is
preferable to add a dispersant such as a surfactant to the
solvent.
[0058] To form the coating layer in the solvent, use can be
preferable made of a method which comprises dissolving the coating
layer material in a solvent wherein the EL phosphor core particles
are dispersed and then adding a reaction solution thereto to
thereby form the coating layer on the particle surface or a method
which comprises simultaneously adding the coating layer material
solution and the reaction solution to the solvent in which the EL
phosphor core particles are dispersed. In such a case, it is
preferable to add the coating layer material solution and the
reaction solution into the region wherein agitation is carried out
most vigorously. To add the coating layer material solution and the
reaction solution, use can be made of a known proportioning pump or
an orifice. It is favorable to use a syringe pump with little
pulsation. In adding the coating layer material solution and the
reaction solution, it is preferable to control the addition speeds
of the individual solutions by detecting ion concentrations in the
reactor. In the case of using urea etc. as the solvent, the
reaction mixture is not restricted to liquid but solid materials
may be added as such.
[0059] The reaction temperature can be controlled by directly
heating the reactor with a mantle heater or the like. However, it
is preferred to control the reaction temperature by providing a
jacket around the reactor and feeding hot water or cold water
thereto. In the case of using water or an organic solvent as the
solvent, the reaction temperature preferably ranges from 40 to
80.degree. C. In the case of using urea as the solvent, the
reaction temperature preferably ranges from 130 to 150.degree. C.
Although the reactions having been discussed so far are to be
carried out under atmospheric pressure, it is also preferable from
the viewpoints of forming a dense coating layer or promoting the
decomposition/condensation reaction to perform the reactions under
elevated pressure with the use of an autoclave. In such a case, the
reaction temperature exceeding 100.degree. C. up to the critical
temperature is usable. To add the solution into the autoclave, it
is preferable to use a feeder pump tolerant to the pressure higher
than the internal pressure of the autoclave.
[0060] A coating layer made of an oxide, a hydroxide, a phosphate,
a fluoride or the like can be formed by the methods as discussed
above. For example, a TiO.sub.2 precursor coating can be formed on
the surface of EL phosphor core particles by adding, as the
reaction solution, about 10 equivalents of an alcohol-diluted water
to a titanium alkoxide. An Mg.sub.3(PO.sub.4).sub.2 coating can be
formed on the surface of EL phosphor core particles by dispersing
the EL phosphor in an aqueous Na.sub.3 (PO.sub.4) solution and
adding an aqueous MgCl.sub.2 solution as the reaction solution.
Also, an MgF.sub.2 coating can be formed on the surface of EL
phosphor core particles by dispersing the EL phosphor in an
alcoholic Mg(CH.sub.3COO).sub.2 solution and adding alcohol-diluted
CF.sub.3COOH as the reaction solution.
[0061] In the two methods of forming a coating layer as described
above, it is also preferable to anneal the thus formed coating
layer. In the case where a hydroxide is partly formed, it can be
almost completely converted into the oxide by annealing.
Furthermore, the denseness of the coating layer can be elevated
thereby and, in its turn, the moisture-proofness and ion-barrier
properties are improved.
[0062] As a third method of forming a coating layer, a method which
comprises mixing EL phosphor core particles with the coating layer
material and applying mechanical and thermal energy thereto to
thereby form a coating layer can be cited.
[0063] Upon the application of the mechanical and thermal energy by
impact or friction, the coating layer material can be solidified on
the surface of the EL phosphor core particles. As an apparatus for
applying the mechanical and thermal energy, use can be preferably
made of a hybridizer, a seater composer, etc. Although an organic
compound such as a polymer resin may be preferably used as the
coating material, it is also possible to use an inorganic compound.
It is also preferable to form an inorganic compound layer on a
coating layer made of an organic compound to give a multilayer
structure or form a coating layer made of a mixture of an organic
compound with an inorganic compound.
[Fabrication of EL Device]
[0064] It is preferable to add the EL phosphor of the invention to
the phosphor layer of an EL device. An EL device has a fundamental
structure wherein a phosphor layer is held between a pair of
electrodes facing each other and at least one of these electrodes
is transparent, It preferably has an adjacent dielectric layer
between the phosphor layer and an electrode. Also, it preferably
has an intermediate layer between the transparent electrode and the
phosphor layer.
[0065] As the phosphor layer, use can be made of a layer wherein
the EL phosphor of the invention (the EL phosphor particles having
the coating layer) is dispersed in a binder. As a binder, it is
possible to use a polymer having a relatively high dielectric
constant or a resin such as a polyethylene-, polypropylene- or
polystyrene-based resin, a silicone resin, an epoxy resin or
vinylidene fluoride. The dielectric constant can be controlled by
adding fine particles having a high dielectric constant (for
example, BaTiO.sub.3 or SrTiO.sub.3) to the binder in an amount of
from 5 to 100 parts by mass per 100 parts by mass of the binder. To
disperse, use can be made of a homogenizer, a sun-and-planet
blender, a roll blender, an ultrasonic disperser or the like.
[0066] The phosphor layer can be formed by applying a coating
solution containing EL phosphor particles. The coating solution
containing EL phosphor particles is a coating solution which
contains at least EL phosphor particles, a binder and a solvent in
which the binder is soluble. As the solvent, use may be made of
acetone, MEK, DMF, butyl acetate, acetonitrile, etc. it is
preferable that the viscosity of the coating solution containing EL
phosphor particles at room temperature is not lower than 0.1 Pas
but not higher than S Pas, still preferably not lower than 0.3 Pas
or higher but not higher than 1.0 Pas. When the viscosity of the
coating solution containing EL phosphor particles is excessively
low, the coating film frequently becomes uneven and the EL phosphor
particles are sometimes separated and sedimented with the passage
of time after the dispersion. When the viscosity of the coating
solution containing EL phosphor particles is excessively high, on
the other hand, coating can be hardly performed at a relatively
high speed in some cases. Accordingly, it is favorable to control
the viscosity within the range as defined above. The term
"viscosity" as used herein means a value measured at 16.degree. C.,
i.e., being the same as the coating temperature.
[0067] It is preferable that the phosphor layer of the EL device of
the invention is formed by continuously coating on a plastic
support provided with a transparent electrode or a laminate having
a back electrode and a dielectric layer, which can be formed if
necessary as will be discussed hereinafter, with the use of a slide
coater, an extrusion coater, a doctor blade coater, etc. In this
step, it is preferable to regulate the thickness variation of the
phosphor layer to 12.5% or less, particularly 5% or less. From the
viewpoint of achieving both of a high luminous efficiency and a
high brightness, the thickness of the phosphor layer, in which the
phosphor particles as described above are employed, preferably
ranges from 40 .mu.m to 100 .mu.m, still preferably form 50 to 80
.mu.m. By controlling the thickness of the phosphor layer as
discussed above, the device can be operated at less power
consumption to give the same brightness and, in its turn, heat
generation accompanying light emission can be reduced. Thus, the EL
device can be prevented from deterioration, thereby giving an EL
device with a high durability.
[0068] Although the packing rate of the EL phosphor particles in
the phosphor layer is not particularly restricted, it is preferably
not less than 60% by mass but not more than 95% by mass, still
preferably not less than 80% by mass but not more than 90% by mass.
In the invention, the particle size of the EL phosphor particles is
regulated to 20 .mu.m or less so that the evenness in the thickness
of the phosphor layer coating film is improved and the smoothness
of the coating film surface is simultaneously improved. Further,
the particle count per unit area can be largely elevated, which
largely relieves fine unevenness in light emission.
[0069] It is preferred that the dispersion type EL device of the
invention is provided with a dielectric layer in addition to the
electrodes and the phosphor layer. The dielectric layer is
preferably located between the phosphor layer and the back
electrode adjacent to the phosphor layer. The dielectric layer can
be formed by using any dielectric material so long as it has a high
electric constant, high insulating properties and a high dielectric
breakage voltage. Such a material is selected from among metal
oxides and nitrides. For example, use can be made of TiO.sub.2,
BaTiO.sub.3, SrTiO.sub.3, PbTiO.sub.3, KNbO.sub.3, PbNbO.sub.3,
Ta.sub.2O.sub.3, BaTa.sub.2O.sub.6, LiTaO.sub.3, Y.sub.2O.sub.3,
Al.sub.2O.sub.3, ZrO.sub.2, AlON, ZnS, etc. These materials may be
provided as either a filmy crystal layer or a membrane having a
particulate structure.
[0070] In the invention, the dielectric layer may be formed in one
side of the phosphor layer. Alternatively, it is also preferable to
form the dielectric layers in both sides of the phosphor layer. In
the case of forming the dielectric layer by coating, use may be
preferably made of a slide coater, an extrusion coater, a doctor
blade coater, etc. as in the formation of the phosphor layer. In
the case of a filmy crystal layer, it may be either a film formed
by sputtering or a gas phase method such as vacuum vapor deposition
or a sol-gel film formed with the use of an alkoxide of Ba or Sr.
In this case, the thickness is usually ranges not less than 0.1
.mu.m but not more than 10 .mu.m. In the case of a particulate
structure, it is preferable that the particle size is sufficiently
small compared with the EL phosphor particle size. More
specifically speaking, it is preferable that the particle size
corresponds to 1/1000 to 1/3 of the EL phosphor particle size.
[0071] It is preferable to form the dielectric layer by applying a
coating solution containing dielectric particles. The coating
solution containing dielectric particles is a coating solution
which contains at least dielectric particles, a binder and a
solvent in which the binder is soluble. Examples of the binder are
the same as those cited above as binders usable in the phosphor
layer. As the solvent, use may be made of acetone, MEK, DMF, butyl
acetate, acetonitrile, etc. it is preferable that the viscosity of
the coating solution containing dielectric particles at room
temperature is not lower than 0.1 Pas but not higher than 5 Pas,
still preferably not lower than 0.3 Pas or higher but not higher
than 1.0 Pas. When the viscosity of the coating solution containing
dielectric particles is excessively low, the coating film
frequently becomes uneven and the dielectric particles are
sometimes separated and sedimented with the passage of time after
the dispersion. When the viscosity of the coating solution
containing dielectric particles is excessively high, on the other
hand, coating can be hardly performed at a relatively high speed in
some cases. Accordingly, it is favorable to control the viscosity
within the range as defined above. The term "viscosity" as used
herein means a value measured at 16.degree. C., i.e., being the
same as the coating temperature.
[0072] As the transparent electrode layer preferably usable in the
EL device of the invention, use may be made of an electrode which
is formed by using any material commonly employed in transparent
electrodes. As examples of such a transparent electrode material,
oxides such as ITO (indium tin oxide), ATO (antimony-doped tin
oxide), ZTO (zinc-doped tin oxide), AZO (aluminum-doped zinc oxide)
and GZO (gallium-doped zinc oxide), a multilayer structure
comprising a silver film sandwiched between highly refractive
layers, .pi.-conjugated polymers such as polyaniline and
polypyrrole, and so on may be cited. It is also favorable that such
a transparent electrode layer is provided with metal wires of the
comb or grid type to improve its electrical conductivity.
[0073] As the transparent electrode layer to be used in the EL
device of the invention, use may be made of an electrode which is
formed on one face of a transparent polymer film with the use of an
arbitrary transparent electrode material. As the polymer film, PET,
PAR, PES or the like may be used. The thickness of the polymer film
is preferably from 20 to 200 .mu.m, still preferably from 50 to 100
.mu.m. It is preferable that the thickness variation is 10% or less
based on the average thickness, still preferably 5% or less.
Examples of the transparent electrode material include oxide films
made of, for example, ITO (indium tin oxide), IZO (indium zinc
oxide), ATO (antimony-doped tin oxide), ZTO (zinc-doped tin oxide),
AZO (aluminum-doped zinc oxide), GZO (gallium-doped zinc oxide) and
FTO (fluorine-doped tin oxide), coating films having an
electrically conductive ink coating in which particles of such an
oxide are dispersed in a polymer, a multilayered film comprising a
silver film sandwiched between highly refractive layers,
.pi.-conjugated polymer films made of, for example, polyaniline and
polypyrrole, and so on. It is preferable from the viewpoint of
exhibiting a high brightness of an EL device that the surface
resistivity of the transparent electrode layer is
300.OMEGA./.quadrature. or less, still preferably
100.OMEGA./.quadrature. or less and still preferably
30.OMEGA./.quadrature. or less. Surface resistivity can be measured
in accordance with the method specified in JIS K6911. The
transmittance at 550 nm of the transparent electrode is preferably
70% or more, still preferably 80% or more and particularly
preferably 90% or more. With an increase in the thickness of the
transparent electrode layer, the surface resistivity is lowered and
the light transmittance is also lowered. Taking the balance between
the electrical conductivity and transmittance, therefore, the
thickness of the transparent electrode layer is preferably from 5
to 500 nm, still preferably from 10 to 300=m.
[0074] It is also favorable that such a transparent electrode layer
is provided with metal wires of the net, comb or grid type to
improve its electrical conductivity and transparency. Although the
transmittance is lowered by providing metal wires, the thickness of
the transparent electrode layer can be reduced thereby. Namely, the
transmittance can be improved exceeding the level of compensating
the lowering by the metal wires, Concerning the metal wire
material, use can be preferably made of copper, gold, silver,
aluminum, nickel, an alloy containing the same or the like. A
material having a high electrical conductivity and a high heat
conductivity is preferred. The metal wire width is preferably from
0.1 to 1000 .mu.m. It is preferable that these metal wires are
located at intervals of from 50 .mu.m to 5 cm, still preferably
from 100 .mu.m to 1 cm. The metal wire height (thickness) is
preferably from 0.1 to 10 .mu.m, still preferably from 0.5 to 5
.mu.m. The metal wire width preferably corresponds to 1/10000 to
1/10 of the intervals among wires. Although the same applies to the
wire height, it preferably corresponds to 1/100 to 10 times of the
wire width. Either the metal wires or the transparent conductive
film may serve as the surface. The smoothness of the conductive
face is preferably 5 .mu.m or less, still preferably from 0.05 to 3
.mu.m. The smoothness of the conductive face means the average
amplitude in the unevenness measured in a section (5 mm.times.5 mm)
with the use of a three-dimensional surface roughness meter (for
example, SURFCOM 575A-3DF manufactured by ACCRETECH). In the case
where the analysis with the surface roughness meter is impossible,
the smoothness is determined by observing under an STM or an
electron microscope.
[0075] To prevent a lowering in voltage with an increase in the EL
device area, it is preferable to form a bus electrode in the inner
periphery on the transparent electrode layer with the use of a
conductivity paste containing fine conductive particles of copper,
gold, silver, carbon, etc. The bus electrode preferably has an area
corresponding to 1% or more of the phosphor layer, still preferably
2% or more so as to efficiently supply power to the phosphor layer.
Since the bus electrode area should be enlarged with an increase in
the phosphor layer area, it is necessary to indicate the area ratio
to the total area of the phosphor layer. The ratio of the bus
electrode preferably area should be 1% or more to achieve a high
brightness by reducing the phosphor layer thickness and elevating
the operation voltage and the frequency. However, it is undesirable
that the bus electrode area ratio exceeds 10%. This is because the
performance of the EL device is not affected thereby but a
non-light emitting member is unnecessarily increased or the device
area is enlarged. To form the bus electrode, use can be made of the
screen printing method or the casting method.
[0076] The back electrode layer, which corresponds to the side form
which no light is taken out, may be formed by using an arbitrary
material having electrical conductivity (a material commonly
employed in forming back electrodes of this type). It may be formed
by coating an electrically conductive paste comprising fine
electrically conductive particles dispersed in a binder or bonding
metallic materials such as copper, aluminum, gold, silver, etc. It
is preferable that the metallic material to be bonded is in the
form of a sheet. As a substitute for the metal sheet, use can be
also made of a graphite sheet. The back electrode preferably has a
heat conductivity of 100 W/mK or above, still preferably 200 W/mK
or above.
[0077] In the case of forming both electrodes by the coating
method, the coating can be carried out with the use of a slide
coater, an extrusion coater, a doctor blade coater or the like as
described above.
[0078] It is preferable that the EL device of the invention has at
least one intermediate layer between the transparent electrode
layer and the phosphor layer. The intermediate layer may be made of
an organic polymer compound, an inorganic compound or a complex
thereof. It is preferable that the EL device has at least one layer
containing an organic polymer compound. The thickness of the
intermediate layer is preferably from 10 nm to 100 .mu.m, still
preferably 100 nm or more but not more than 30 .mu.m and
particularly preferably 0.5 .mu.m or more but not more than 10
.mu.m.
[0079] In the case of using an organic polymer compound as a
material for forming the intermediate layer, examples of the
polymer compound usable therefor include polyethylene,
polypropylene, polystyrene, polyesters, polycarbonates, polyamdies,
polyether sulfones, polyvinyl alcohol, polysaccharides such as
pullulan, saccharose and cellulose, vinyl chloride, fluorinated
rubber, polyacrylates, polymethacrylates, polyacrylic amides,
polymethacrylic amides, silicone resin, cyanoethylpullulan,
cyanoethyl polyvinyl alcohol, cyanoethylsaccharose, UV-curable
resins obtained from polyfunctional acrylate compounds,
heat-curable resins obtained from epoxy compounds or cyanate
compounds and so on. The polymer compound to be used herein may be
either an electrically insulating material or an electrically
conductive material.
[0080] Such an organic polymer compound or its precursor may be
dissolved in an appropriate organic solvent and then coating on the
transparent electrode or the phosphor layer to form the
intermediate layer. In this case, it is preferable to carry out the
coating with the use of a slide coater, an extrusion coater, a
doctor blade coater or the like as described above. Examples of the
organic solvent include dichloromethane, chloroform, acetone,
methyl ethyl ketone, cyclohexanone, acetonitrile,
dimethylformamide, dimethylacetamide, dimethyl sulfoxide, toluene,
xylene and so on.
[0081] The intermediate layer may contain additives) for imparting
various functions so long as it remains substantially transparent.
The transmittance at 550 nm of the intermediate layer is preferably
70% or more, still preferably 80% or more. For example, it may
contain a dielectric such as barium titanate particles, an
electrically conductive material such as tin oxide, indium oxide,
tin oxide-indium or metal particles, a dye, a fluorescent dye or a
fluorescent pigment. Moreover, it may contain light-emitting
particles to such an extent that the advantage of the invention is
not damaged thereby (i.e. in an amount attaining not more than 30%
of the brightness of the total electroluminescent phosphor).
[0082] The intermediate layer may be made of an inorganic compound
such as SiO.sub.2, another metal oxide or a metal nitride. To form
the intermediate layer with the use of an inorganic compound, it is
possible to employ the sputtering method, the CVD method, etc. In
the case of forming the intermediate layer with the use of an
inorganic compound, the thickness is preferably more than 10 nm but
not more than 1 .mu.m, still preferably more than 10 nm but not
more than 200 nm. It is also favorable that the intermediate layer
is composed of an inorganic compound layer and an organic polymer
compound layer.
[0083] It is preferable that the EL device of the invention has at
least one intermediate layer containing an organic polymer compound
and having a thickness of 0.5 .mu.m or more but not more than 10
.mu.m. It is preferable that the organic polymer compound is one
selected from among polyesters, polycarbonates, polyamdies,
polyether sulfones, fluorinated rubbers, polyacrylates,
polymethacrylates, polyacrylic amides, polymethacrylic amides,
silicone resins, cyanoethylpullulan, cyanoethyl polyvinyl alcohol,
cyanoethylsaccharose, UV-curable resins obtained from
polyfunctional acrylate compounds and heat-curable resins obtained
from epoxy compounds or cyanate compounds and so on. Among these
compounds, one having a softening point of 70.degree. C. or above
(still preferably 100.degree. C. or above) is preferred. It is also
preferable to use a combination of two or more polymer compounds
selected from those cited above.
[0084] In the case where the organic polymer compound employed in
the intermediate layer has a high softening point (for example,
200.degree. C. or above), it is also preferred to use another
intermediate layer containing an organic polymer compound having a
lower softening point so as to improve the adhesiveness to the
transparent electrode layer of the phosphor particle-containing
layer.
[0085] To achieve white light emission, a red light-emitting
material is employed together with bluish green light-emitting zinc
sulfide particles in the electroluminescent device of the
invention. The red light-emitting material may be dispersed in the
phosphor particle layer. Alternatively, it may be dispersed in the
dielectric layer. It may be provided either between the phosphor
particle layer and the transparent electrode or in the opposite
side to the phosphor particle layer concerning the transparent
electrode.
[0086] In the electroluminescent device of the invention, the light
emission wavelength in emitting white light is preferably 600 nm or
more but not more than 650 nm. To obtain red light wavelength
falling within this range, the red light-emitting material may be
contained in the phosphor particle layer, or provided between the
phosphor particle layer and the transparent electrode or in the
opposite side to the phosphor particle layer concerning the
transparent electrode. It is most preferable that the red
light-emitting material is contained in the dielectric layer.
Although it is preferable that the whole dielectric layer in the
electroluminescent device of the invention serves a dielectric
layer containing the red light-emitting material, it is more
preferable that the dielectric layer in the device is divided in
two or more layers and a part thereof serves as a layer containing
the red light-emitting material. It is preferable that the layer
containing the red light-emitting material is provided between the
dielectric layer and the phosphor particle layer. It is also
preferred that the layer containing the red light-emitting material
is sandwiched between dielectric layers free from the red
light-emitting material. In the case where the layer containing the
red light-emitting material is located between the dielectric layer
free from the red light-emitting material and the phosphor particle
layer, the thickness of the layer the red light-emitting material
is preferably 1 .mu.m or more but not more than 20 .mu.m, still
preferably 3 .mu.m or more but not more than 17 .mu.m. The
concentration of the red light-emitting material in the dielectric
layer containing the red light-emitting material is preferably 1%
by weight or more but not more than 20% by weight, still preferably
3% by weight or more but not more than 15% by weight, based on the
dielectric particles. In the case where the layer containing the
red light-emitting material is sandwiched between dielectric layers
free from the red light-emitting material, the thickness of the
layer containing that the layer containing the red light-emitting
material is preferably 1 .mu.m or more but not more than 20 .mu.m,
still preferably 3 .mu.m or more but not more than 10 .mu.m. The
concentration of the red light-emitting material in the dielectric
layer containing the red light-emitting material is preferably 1%
by weight or more but not more than 30% by weight, still preferably
3% by weight or more but not more than 20% by weight, based on the
dielectric particles. In the case where the layer containing the
red light-emitting material is sandwiched between dielectric layers
free from the red light-emitting material, it is also preferable
that the layer containing the red light-emitting material is free
from dielectric particles but composed exclusively of a highly
dielectric binder and the red light-emitting material.
[0087] The light emission wavelength of the red light-emitting
material to be used herein in the form of a powder is preferably
600 nm or more but not more than 750 nm, still preferably 610 nm or
more but not more than 650 nm and most preferably 610 nm or more
but not more than 630 nm. This light-emitting material is added to
the electroluminescent device. The red light emission wavelength at
the electroluminescent light emission is preferably 600 nm or more
but not more than 650 nm, still preferably 605 nm or more but not
more than 630 nm and most preferably 608 nm or more but not more
than 620 nm.
[0088] When such a red light-emitting material layer is formed in
the dielectric layer, the total thickness of the dielectric layer
is preferably 5 .mu.m or more but not more than 40 .mu.m, still
preferably 10 .mu.m or more but not more than 35 .mu.m. The
dielectric particles to be used in the dielectric layer containing
the red light-emitting material can be selected from the same
particles as in the dielectric layer free from red light-emitting
material. The dielectric layer particles in the layer containing
the red light-emitting material may be either the same or different
from the particles in the layer free from red light-emitting
material. As a binder in the layer containing the red
light-emitting material, it is preferable to employ a polymer
having a relatively high dielectric constant such as a cyanoethyl
cellulose-based resin or a resin such as polyethylene,
polypropylene, a polystyrene-based resin, a silicone resin, an
epoxy resin or vinylidene fluoride. To disperse the dielectric
material, use may be preferably made of a homogenizer, a
sun-and-planet blender, a roll blender, an ultrasonic disperser or
the like.
[0089] As the red light-emitting material in the invention, use can
be preferably made of a fluorescent pigment or a fluorescent dye.
As a compound serving as the emission center, a compound having a
rhodamine, lactone, xanthene, quinoline, benzothiazole,
triethylindoline, perylene, triphenin or dicyanomethylene skeleton
is preferred. It is also preferable to use a cyanine pigment, an
azo dye, a polyphenylene vinylene-based polymer, a disilane
oligoethienylene polymer, a ruthenium complex, a europium complex
or an erbium complex. Either one or two or more of these compounds
may be used. It is also possible to disperse such a compound in a
polymer or the like before using.
[0090] It is preferable to continuously form each of the layers as
discussed above at least from coating to drying. The drying step is
divided into a constant-rate drying step wherein drying is
performed until the coating film is dried and solidified and a
decreasing drying step wherein the solvent remaining in the coating
film is reduced. When the drying is effected at a high speed in the
invention wherein each layer has a high binder content, the surface
is exclusively dried and a convective flow arises in the coating
film, thereby frequently causing the so-called Benard cell
phenomenon. Furthermore, a rapid expansion of the solvent
frequently results in blister troubles which seriously damage the
evenness of the coating film. When the drying temperature is too
low, on the other hand, the solvent remains in each layer, which
affects the subsequent steps of fabricating an EL device such as
the step of laminating a moisture-proof film. In the drying step,
therefore, it is preferable that the constant-rate drying is slowly
carried out and then the decreasing drying is carried out at a
temperature sufficient for drying the solvent. To carry out the
constant-rate drying slowly, it is preferable that a drying room in
which the base runs into several zones is divided into several
zones and the drying temperature is elevated stepwise after the
completion of the coating.
[0091] In the dispersion type EL device of the invention, it is
preferable that a sealing film is used in the final stage to
eliminate the effects of humidity and oxygen from the external
environment.
[0092] The water vapor transmission rate at 40.degree. C.-90% RH,
which is measured in accordance with the method as defined in JIS
K7129, of the sealing film to be used for sealing the EL device is
preferably 0.05 g/m.sup.2/day or less, still preferably 0.01
g/m.sup.2/day or less. It is also preferable that the oxygen
transmission rate thereof at 40.degree. C.-90% RH is 0.1
cm.sup.3/m.sup.2/day/atm or less, still preferably 0.01
cm.sup.3/m.sup.2/day/atm or less. As the sealing film, use may be
preferably made of a laminated film composed of an organic film
with an inorganic film.
[0093] It is preferable that the organic film is made of a
polyethylene-based resin, a polypropylene-based resin, a
polycarbonate-based resin, a polyvinyl alcohol-based resin, etc. A
polyvinyl alcohol-based resin is particularly preferred therefor.
Since a polyvinyl alcohol-based resin or the like has a hygroscopic
nature, it is preferable to absolutely drying it by, for example,
vacuum drying before using. Such a resin is shaped into a sheet by
the coating method or the like and then the inorganic film is
layered thereon by vapor deposition, sputtering, using the CVD
method, etc. The inorganic film to be layered is preferably made of
silicon oxide, silicon nitride, silicon oxynitride, silicon
oxide/aluminum oxide, aluminum nitride and so on. Among all,
silicon oxide is preferably employed therefor. To achieve a lower
water vapor transmittance rate or a lower oxygen transmittance rate
or to prevent the inorganic film from cracking caused by bending,
etc., it is preferable to form a multilayer film by repeatedly
forming organic films and inorganic films or bonding multiple
organic films having inorganic films layered thereon via adhesive
layers. The thickness of the organic film preferably ranges from 5
.mu.m to 300 .mu.m, still preferably from 10 .mu.m to 200 .mu.m.
The thickness of the inorganic film preferably ranges from 10 nm to
300 nm, still preferably from 20 nm to 200 nm. The thickness of the
layered sealing film preferably ranges from 30 .mu.m to 1000 .mu.m,
still preferably from 50 .mu.m to 300 .mu.m.
[0094] In the case of sealing an EL cell with this sealing film,
the EL cell may be sandwiched between two sealing film sheets
followed by sealing the periphery. Alternatively, a single sealing
film may be folded in half followed by sealing the overlapped part.
The EL cell to be sealed with the sealing film may be separately
fabricated. Alternatively, the EL cell may be formed directly on
the sealing film. In this case, the film can serve as a substitute
for the support. It is preferable to perform the sealing step in
vacuo or in a dry atmosphere under dew-point control.
[0095] In addition those discussed above, it is also preferable
that the EL device has a cushioning material layer made of, for
example, a polymer material having an excellent shock-absorbing
function or a foamed polymer material containing a foaming agent
for preventing oscillation, a compensation electrode layer facing
the transparent electrode layer or the back electrode layer across
an insulating layer, and so on.
[0096] The EL device of the invention can be fabricated by using
any of the following methods. Namely, use may be preferably made of
a method comprising bonding a laminate, which has been formed by
successively coating a dielectric layer and a phosphor layer on a
back electrode layer such as an aluminum foil, to a transparent
electrode layer; a method comprising bonding a laminate, which has
been formed by successively coating a phosphor layer and a
dielectric layer on a transparent electrode layer, to a back
electrode layer; a method comprising bonding a laminate, which has
been formed by coating a phosphor layer on a transparent electrode
layer, to another laminate, which has been formed by coating a
dielectric layer on a back electrode layer; and so on. The bonding
is preferably carried out by the heat compression bonding method
with the use of a heat roller coated with a metal, a silicone
resin, etc. The heat compression bonding temperature preferably
ranges from 100 to 300.degree. C., still preferably from 150 to
200.degree. C. The heat compression bonding speed preferably ranges
from 0.01 to 1 m/min, still preferably from 0.05 to 0.5 m/min. The
heat compression bonding pressure preferably ranges from 0.01 to 1
MPa/m.sup.2, still preferably from 0.05 to 0.5 MPa/m.sup.2. When
the heat compression bonding temperature or pressure is too low, a
sufficient adhesion strength cannot be achieved and thus there
arises a tendency toward interlayer peeling. When the heat
compression bonding temperature or pressure is too high, on the
other hand, the phosphor layer or the dielectric layer is
excessively rolled and thinned and there arises a tendency toward
insulation breakdown. In this case, the deterioration in the binder
contained in the phosphor layer or the dielectric layer due to the
high temperature and the disruption of the EL phosphor particles
due to the high pressure are also observed.
[0097] In the dispersion type EL device of the invention, it is
preferable that a sealing film is used in the final stage to
eliminate the effects of humidity and oxygen from the external
environment.
[0098] The water vapor transmission rate at 40.degree. C.-90% RH,
which is measured in accordance with the method as defined in JIS
K7129, of the sealing film to be used for sealing the EL device is
preferably 0.1 g/m.sup.2/day or less, still preferably 0.05
g/m.sup.2/day or less and particularly preferably 0.01
g/m.sup.2/day or less. It is also preferable that the oxygen
transmission rate thereof at 40.degree. C.-90% RH is 0.1
cm.sup.3/m.sup.2/day/atm or less, still preferably 0.01
cm.sup.3/m.sup.2/day/atm or less. As the sealing film, use may be
preferably made of a polytrifluoroethylene chloride resin or a
laminated film composed of an organic film with an inorganic
film.
[0099] In the case of using a laminate film as the sealing film, it
is preferable that the organic film is made of a polyethylene-based
resin, a polypropylene-based resin, a polycarbonate-based resin, a
polyvinyl alcohol-based resin, etc. A polyvinyl alcohol-based resin
is particularly preferred therefor. Since a polyvinyl alcohol-based
resin or the like has a hygroscopic nature, it is preferable to
absolutely drying it by, for example, vacuum drying before using.
Such a resin is shaped into a sheet by the coating method or the
like and then the inorganic film is layered thereon by vapor
deposition, sputtering, using the CVD method, etc. The inorganic
film to be layered is preferably made of silicon oxide, silicon
nitride, silicon oxynitride, silicon oxide/aluminum oxide, aluminum
nitride and so on. Among all, silicon oxide is preferably employed
therefor. To achieve a lower water vapor transmittance rate or a
lower oxygen transmittance rate or to prevent the inorganic film
from cracking caused by bending, etc., it is preferable to form a
multilayer film by repeatedly forming organic films and inorganic
films or bonding multiple organic films having inorganic films
layered thereon via adhesive layers. The thickness of the organic
film preferably ranges from 5 .mu.m to 300 .mu.m, still preferably
from 10 .mu.m to 200 .mu.m. The thickness of the inorganic film
preferably ranges from 10 nm to 300 nm, still preferably from 20 nm
to 200 nm. The thickness of the layered sealing film preferably
ranges from 30 .mu.m to 1000 .mu.m, still preferably from 50 .mu.m
to 300 .mu.m.
[0100] A hot-melt adhesive is applied to one face of the sealing
film. In the case of sealing an EL cell with this sealing film, the
EL cell may be sandwiched between two sealing film sheets followed
by heat compression bonding. Alternatively, a single sealing film
may be folded in half followed by heat compression bonding. In the
heat compression bonding, it is preferable to use a heat roller as
described above, a press type heat compression bonding machine,
etc. The heat compression bonding temperature preferably ranges
from 100 to 200.degree. C. The EL cell to be sealed with the
sealing film may be separately fabricated. Alternatively, the EL
cell may be formed directly on the sealing film. In this case, the
film can serve as a substitute for the support. It is preferable to
perform the sealing step in vacuo or in a dry atmosphere under
dew-point control. In the case of sealing in vacuo, the pressure is
preferably 10.sup.-2 Pa or below.
[0101] In addition those discussed above, it is also preferable
that the EL device has a cushioning material layer made of, for
example, a polymer material having an excellent shock-absorbing
function or a foamed polymer material containing a foaming agent
for preventing oscillation, a compensation electrode layer facing
the transparent electrode layer or the back electrode layer across
an insulating layer, and so on. To effectively eliminate the heat
generated from the EL device by driving, it is also preferable to
provide a radiating sheet having a ceramic material dispersed
therein. Moreover, it is preferable to provide an ultraviolet
ray-absorbing layer to prevent UV-induced color change in the
fluorescent pigment contained in an image sheet as will be
discussed hereinafter or the EL device, or an electromagnetic
wave-absorbing layer to prevent the release of electromagnetic wave
from the EL device.
[0102] In general, an EL device is driven with the use of an AC
source of 50 Hz to 400 Hz at 100 V. In an EL device having a small
area, the brightness increases almost proportionally to the applied
voltage and the frequency.
[0103] In an EL device having a large area of 0.25 m.sup.2 or
larger, however, the capacity component of the EL device is
increased and thus there arises an impedance matching error between
the EL device and the power source or the time constant required
for the charge storage in the EL device is elevated. As a result,
it is frequently observed that sufficient power cannot be supplied
even under a high voltage or a high frequency. In the case of
driving an EL device of 0.25 m.sup.2 or larger with an AC source of
500 Hz or more, in particular, the applied voltage is frequently
lowered with an increase in the driving frequency and, in its turn,
the brightness is lowered. In contrast thereto, the EL device of
the invention can be driven at a high frequency even in the case of
having a large size of 0.25 m.sup.2 or more to thereby achieve a
high brightness. In such a case, the driving is preferably
performed at 500 Hz to 5 KHz, still preferably 800 Hz to 3 KHz.
[0104] As examples of the application of the EL device of the
invention, interior and exterior signs and displays may be cited.
Image display systems with the combined use of the EL device with
transfer imaging sheets such as color photo prints inkjet prints
are particularly preferred. To ensure a favorable image visibility,
the density of such a transfer imaging sheet preferably ranges from
1.5 to 4.5, still preferably from 2 to 3. The transfer imaging
sheet is closely bonded to the light-emitting face of the EL
device. The transfer imaging sheet may be bonded by using pressure,
static electricity, etc. Alternatively, it may be detachably bonded
to the EL device with an adhesive or the like. To elevate the
whiteness in non-light emission, it is also preferable to provide a
diffuser sheet, etc. between the image sheet and the EL device.
Moreover, the surface of the image sheet may be provided with a
protective sheet made of a resin. To ensure sufficient light
resistance, impact resistance and transparency, it is preferable to
use a resin such as an acrylic resin or a polycarbonate resin,
which may be provided with an UV-absorbing layer, as the protective
sheet. To ensure a sufficient rigidity and prevent the EL device
from damages by a cutter knife or an edged metallic tool or
electrical shock, it is preferable that the protective sheet has a
thickness of form 1 to 10 mm, still preferably from 2 to 8 mm. It
is preferable that an image display unit made up of the EL device,
the image sheet and the protective layer is fixed to a fixing
member comprising a fixing frame and a backboard made of aluminum,
a resin, a wood material, etc. The EL device may be detachably
fixed to the fixing frame or the backboard by using an adhesive or
the like. Alternatively, it may be fixed by pressuring, etc. In the
case of fixing to a curved face such as a column, it is preferable
to use a fixing member having the same curvature as in the face to
be fixed. From the viewpoint of space-saving, it is also preferred
that the power source of the EL device is contained a room formed
in the fixing member.
EXAMPLES
[0105] To further illustrate the EL phosphors of the invention,
methods of producing the same and EL devices in greater detail, the
following EXAMPLES will be given. However, it is to be understood
that the embodiments of the invention are not restricted to these
EXAMPLES.
[Production of EL Phosphor Particles]
<EL Phosphor Particles A>
[0106] As a ZnS materials, ZnS having a crystallite size of 20 nm
and an average particle size of 2 .mu.m was prepared. A 25 g
portion of this ZnS was weighed and fed together with 200 ml of
distilled water into a 300 ml beaker. Then the mixture was stirred
with a magnet stirrer until all ZnS particles were dispersed.
[0107] A 0.064 g portion of CsSO.sub.4.5H.sub.2O was weighed and
dissolved in 2 ml of distilled water to give an aqueous solution.
This solution was added to the dispersion of the ZnS particles as
described above within about 30 seconds by using a burette.
Stirring was continued for 30 minutes after the completion of the
addition. After ceasing the stirring, the mixture was allowed to
stand until the ZnS particles were sedimented. After the complete
sedimentation of the ZnS particles, the supernatant was removed by
decantation. Then 200 ml of distilled water was added for washing
and the particles were redispersed by stirring. After stirring for
10 minutes, the ZnS particles were sedimented and the supernatant
was removed by decantation. After repeating this washing procedure
thrice, the particles were dried with a hot-air dryer at
120.degree. C. for 4 hours to give ZnS containing Cu added
thereto.
[0108] To the Cu-containing ZnS, the following flux and additives
were added and mixed in a mortar t give a mixture. TABLE-US-00002
Cu-containing ZnS 25 g Sodium chloride 0.5 g Barium chloride
dihydrate 1.0 g Magnesium chloride hexahydrate 2.1 g
[0109] This mixture was filled into an alumina crucible and
covered. Then it was put in a Maffle oven at room temperature. The
Maffle oven was heated at a speed of 800.degree. C./h and then
maintained at 1250.degree. C. Thus, first baking was carried out in
the atmosphere for 1 hour. After the completion of the first
baking, the product was allowed to spontaneously cool to room
temperature and then the alumina crucible was taken out. The
firstly baked mixture was taken out from the alumina crucible,
washed with 500 ml of a 0.1 M aqueous HCl solution, then washed
with 500 ml of distilled water 5 times and dried with a hot air
dryer at 120.degree. C. for 4 hours. Thus, intermediate phosphor
particles (ZnS:Cu, Cl) were obtained.
[0110] 5 g of the intermediate phosphor particles and 20 g of
alumina balls (1 mm) were packed in a glass bottle (diameter: 15
mm) and the particles were ball-milled at 10 rpm for 20 minutes.
Then the alumina balls were separated from the intermediate
phosphor particles by using a 100-mesh sieve. The intermediate
phosphor particles thus separated were filled in an alumina
crucible and covered. Then it was put in a Maffle oven at room
temperature. The Maffle oven was heated at a speed of 400.degree.
C./h and then maintained at 700.degree. C. Thus, second baking was
carried out in the atmosphere for 4 hours. After the completion of
the second baking, the product was allowed to cool to room
temperature and then the alumina crucible was taken out. The
secondly baked mixture was taken out from the alumina crucible,
washed with 100 ml of a 10% aqueous KCN solution, then washed with
500 ml of distilled water 5 times and dried with a hot air dryer at
120.degree. C. for 4 hours. Thus, EL phosphor particles A
(ZnS:Cu,Cl) were obtained.
<EL Phosphor Particles B>
[0111] As in the EL phosphor particles A as described above, ZnS
containing Cu added thereto was prepared.
[0112] To the Cu-containing ZnS, the following fluxes were added
and mixed in a mortar t give a mixture. TABLE-US-00003
Cu-containing ZnS 25 g Strontium chloride hexahydrate 27.3 g Barium
chloride dihydrate 4.2 g Magnesium chloride hexahydrate 11.1 g
Chloroaurate tetrahydrate 0.0053 g
[0113] Subsequently, the procedures employed in producing the EL
phosphor particles A were followed to give EL phosphor particles B
(ZnS:Cu,Cl,Au).
<EL Phosphor Particles B'>
[0114] The procedures employed in producing the EL phosphor
particles B were followed but not adding "chloroaurate
tetrahydrate" to give EL phosphor particles B' (ZnS;Cu,Cl).
<EL Phosphor Particles B''>
[0115] The procedures employed in producing the EL phosphor
particles B were followed with adding not 0.0053 g but 0.1 g of
"chloroaurate tetrahydrate" to give EL phosphor particles B''
(ZnS:Cu,Cl,Au).
<EL Phosphor Particles B1>
[0116] The procedures employed in producing the EL phosphor
particles B were followed but not adding "chloroaurate
tetrahydrate" but adding 17.5 Mg of Na.sub.2[Pt(OH).sub.6] to
"barium chloride dihydrate", well mixing and then further mixing
with the ZnS particles and other fluxes followed by baking the
mixture, to give EL phosphor particles B3 (ZnS:Cu,Cl,Pt).
<EL Phosphor Particles B2>
[0117] The procedures employed in producing the EL phosphor
particles B were followed but adding 17.5 mg of
Na.sub.2[Pt(OH).sub.6] to "barium chloride dihydrate", well mixing
and then further mixing with the ZnS particles and other fluxes
followed by baking the mixture, to give EL phosphor particles B2
(ZnS:Cu,Cl,Au,Pt).
<EL Phosphor Particles C>
[0118] The procedures employed in producing the EL phosphor
particles B' were followed but adding 0.03 g of antimony
trichloride to the intermediate phosphor particles of the EL
phosphor particles B' to give EL phosphor particles C
(ZnS:Cu,Cl,Sb).
<EL Phosphor Particles D>
[0119] The procedures employed in producing the EL phosphor
particles B' were followed but adding 0.04 g of bismuth trichloride
to the intermediate phosphor particles of the EL phosphor particles
B, to give EL phosphor particles D (ZnS:Cu,Cl,Bi).
<EL Phosphor Particles E>
[0120] As in the EL phosphor particles A as described above, ZnS
containing Cu added thereto was prepared.
[0121] To the Cu-containing ZnS, the following fluxes were added
and mixed in a mortar t give a mixture. TABLE-US-00004
Cu-containing ZnS 25 g Strontium chloride hexahydrate 27.3 g Barium
chloride dihydrate 4.2 g Magnesium chloride hexahydrate 11.1 g
Cesium chloride 4.5 g
[0122] Subsequently, the procedures employed in producing the EL
phosphor particles A were followed to give EL phosphor particles E
(ZnS:Cu,Cl,Cs).
<EL Phosphor Particles F>
[0123] The procedures employed in producing the EL phosphor
particles B were followed but adjusting the first baking
temperature to 1100.degree. C. to give EL phosphor particles F
(ZnS:Cu, Cl).
<EL Phosphor Particles G>
[0124] The procedures employed in producing the EL phosphor
particles A were followed but omitting the ball mill treatment of
the intermediate phosphor particles to give EL phosphor particles G
(ZnS:Cu,Cl).
[Evaluation of Particles]
[0125] The EL phosphor particles A to G were evaluated in the
following items. Table 1 summarizes the results.
[0126] Average particle size (using the median diameter calculated
by using LA-920 manufactured by HORIBA)
[0127] Coefficient of variation in particle size distribution
(using the coefficient of variation calculated by using LA-920
manufactured by HORIBA)
[0128] Interplanar spacing in stacking fault (grinding phosphor
particles in an agate mortar, observing fragments under a TEM,
measuring the maximum interplanar spacings in stacking faults and
counting sheets)
[0129] stacking fault frequency (observing 100 fragments obtained
above under a TEM and measuring the frequency of stacking
faults)
[0130] Phosphor composition analysis (quantifying Au and Pt by
ICP-MAS)
[0131] As Table 1 indicates, the EL phosphor particles A to E each
contained particles having 10 or more stacking faults with
interplanar spacings of 5 nm or less in an amount of 50% or more,
while the EL phosphor particles G had a large interlayer spacing
and a frequency less than 30%. TABLE-US-00005 TABLE 1 Average
Coefficient Interplanar Stacking Amount of Au Amount of Pt particle
of spacing in No. of fault added added EL phosphor size variation
stacking stacking frequency (mol/mol (mol/mol particle (.mu.m) (%)
fault (nm) faults (%) ZnS) ZnS) A 24.5 42.0 4 10< 65 0 0 B 17.7
34.0 4 10< 80 2 .times. 10.sup.-6 0 B' 17.3 33.2 4 10< 78 0 0
B'' 17.6 33.8 4 10< 77 6 .times. 10.sup.-4 0 B1 17.4 33.6 4
10< 81 0 2 .times. 10.sup.-5 B2 17.2 33.1 4 10< 79 2 .times.
10.sup.-6 2 .times. 10.sup.-5 C 17.2 33.5 4 10< 76 0 0 D 16.7
33.7 4 10< 71 0 0 E 16.0 33.8 4 10< 78 0 0 F 8.7 31.9 4
10< 60 2 .times. 10.sup.-6 0 G 17.9 34.2 8 10< 25 0 0
[Formation of Coating Layer] <Coated EL Phosphor Particles A to
G>
[0132] Using the EL phosphor particles A to G, a coating layer made
of TiO.sub.2 was formed on the surface of the particles by using a
fluidized bed reactor shown in FIG. 1. The fluidized bed reactor
has a cylindrical reaction tank 7 provided with a perforated plate
8 at the bottom. The reaction tank is surrounded by a heater 9 and
thus temperature-controlled. To the lower part of the perforated
plate 9, a line 10 is connected for supplying a carrier gas for
fluidizing the EL phosphor particles 1 and a gaseous coating layer
material. A reaction gas inlet pipe 12, which is connected to a
line for supplying a reaction gas, is provided close to the
perforated plate in the reaction tank. These gas-supplying lines
are also heated by the heater and provided, individually at the
intermediate parts, with storage tanks 13 and 14 for vaporizing a
coating layer material 2 and a reactant 3 respectively. The coating
layer material 2 and the reactant 3 stored in the storage tanks are
vaporized by bubbling with the carrier gases 4 and 5. The unreacted
gases discharged from a reaction tank or the by-product gas 6 are
discharged via an exhaust duct 15 connected to a scrubber (not
shown).
[0133] 100 g of each of the EL phosphor particles A to G were
packed in the reaction tank of the fluidized bed reactor. In the
case of the EL phosphor particles F, fluidization could not be
sufficiently carried out by using the EL phosphor particles F
alone. Thus, 50 g of spherical alumina particles having an average
particle size of 25 .mu.m were added as a fluidization promoter to
50 g of the EL phosphor particles F. As the coating layer material,
a TiCl.sub.4 solution kept at 35.degree. C. was fed into the
storage tank. As the reactant, distilled water kept at 30.degree.
C. was fed into the storage tank. As the carrier gas, Ar was fed
via the perforated plate at a flow rate of 500 cc/min to thereby
fluidize the EL phosphor particles. After heating the reaction tank
to 200.degree. C., bubbling of the TiCl.sub.4 was initiated with
the use of the Ar gas. At the same time, bubbling of the distilled
water was also initiated with the use of the Ar gas. The Ar gas was
supplied at a flow rate of 300 cc/min in both cases. After 2 hours,
the gas supply was ceased and the reaction tank was cooled. Then
the EL phosphor particles were collected to give coated EL phosphor
particles A to G. Each of the EL phosphor particles thus collected
had a TiO.sub.2 coating layer having an average thickness of 150 nm
on the surface.
<Coated EL Phosphor Particles b>
[0134] Using the EL phosphor particles B, the above procedures were
followed but carrying out the bubbling not for 2 hours but for 10
minutes to thereby form a coating layer on the surface of the EL
phosphor particles. Thus, coated EL phosphor particles b were
obtained. The EL phosphor particles thus collected had a TiO.sub.2
coating layer having an average thickness of 10 nm on the
surface.
<Coated EL Phosphor Particles H>
[0135] Using the EL phosphor particles B, the above procedures were
followed but replacing TiCl.sub.4 in the coating layer A by
trimethylaluminum and also replacing the reaction gas by O.sub.2 to
thereby form a coating layer on the surface of the EL phosphor
particles. Thus, coated EL phosphor particles H were obtained. The
EL phosphor particles thus collected had an Al.sub.2O.sub.3 coating
layer having an average thickness of 170 nm on the surface.
<Coated EL Phosphor Particles I>
[0136] Using the EL phosphor particles Be the above procedures were
followed but replacing TiCl.sub.4 in the coating layer A by
hexadimethylamide dialuminum and also replacing the reaction gas by
NH.sub.3 to thereby form a coating layer on the surface of the EL
phosphor particles. Thus, coated EL phosphor particles I were
obtained. The EL phosphor particles thus collected had an AlN
coating layer having an average thickness of 110 nm on the
surface.
<Coated EL Phosphor Particles J>
[0137] Using the EL phosphor particles B, a coating layer made of
SiO.sub.2 was formed on the surface of the particles by using an
agitated bed reactor shown in FIG. 2. The agitated bed reactor has
a cylindrical reaction tank 17 having an agitator 18 therein. The
reaction tank is surrounded by a heater 19 and thus
temperature-controlled. At the bottom of the reaction tank, lines
20, 21 and 22 respectively for supplying an auxiliary carrier gas
for fluidizing the EL phosphor particles 1, a gaseous coating layer
material and a reaction gas are attached. These gas-supplying lines
are also heated by the heater and provided, individually at the
intermediate parts, with storage tanks 23 and 24 for vaporizing a
coating layer material 2 and a reactant 3 respectively. The coating
layer material 2 and the reactant 3 stored in the storage tanks are
vaporized by bubbling with the carrier gases 4 and 5. The unreacted
gases discharged from a reaction tank or the by-product gas 6 are
discharged via an exhaust duct 25 connected to a scrubber (not
shown).
[0138] 100 g of the EL phosphor particles B were packed in the
reaction tank of the agitated bed reactor. As the coating layer
material, an SiCl.sub.4 solution kept at 35.degree. C. was fed into
the storage tank. As the reactant, distilled water kept at
30.degree. C. was fed into the storage tank. As the auxiliary
carrier gas, Ar was supplied through the auxiliary gas-supplying
pipe at a flow rate of 200 cc/min and the paddle type agitator was
rotated at 30 rpm to thereby fluidize the EL phosphor particles.
After heating the reaction tank to 200.degree. C., bubbling of the
SiCl.sub.4 was initiated with the use of the Ar gas. At the same
time, bubbling of the distilled water was also initiated with the
use of the Ar gas. The Ar gas was supplied at a flow rate of 300
cc/min in both cases. After 2 hours, the gas supply was ceased and
the reaction tank was cooled. Then the EL phosphor particles were
collected to give coated EL phosphor particles J. The EL phosphor
particles thus collected had an SiO.sub.2 coating layer having an
average thickness of 100 nm on the surface.
<Coated EL Phosphor Particles K>
[0139] Using the EL phosphor particles B, a coating layer made of
Ta.sub.2O.sub.3 was formed on the surface of the particles by using
a vibrated bed reactor shown in FIG. 3. The vibrated bed reactor
has a horizontally located phosphor-containing part 27 which is
vibrated by a vibration generator 28 to thereby fluidize EL
phosphor particles 1. The phosphor-containing part 27 is tightly
closed by a reaction tank 29 and surrounded by a heater 30 and thus
temperature-controlled. A coating layer material 2 is supplied in
the form of a liquid from a coating material-supplying nozzle 31 by
a feeder pump 32. Then the coating layer material reacts with a
carrier gas 4 and a reaction gas 26 supplied from gas-supplying
lines. The unreacted gases discharged from a reaction tank or the
by-product gas 6 are discharged via an exhaust duct connected to a
scrubber (not shown).
[0140] 100 g of the EL phosphor particles a were packed in the
phosphor-containing part of the vibrated bed reactor. By using an
unbalance mass type vibrator, the phosphor-containing layer was
vibrated at 1 KHz to thereby fluidize the EL phosphor particles. As
the carrier gas, N.sub.2 was supplied at a flow rate of 200 cc/min.
After heating the reaction tank to 400.degree. C., a 0.1 wt %
ethanol solution of TaCl.sub.5 was sprayed from the coating layer
material-supplying nozzle onto the fluidized EL phosphor particles
at a rate of 100 cc/min. After spraying for 10 minutes, the
spraying was ceased and the particles were dried for 10 minutes.
Next, the carrier gas was replaced by O.sub.2 employed as the
reaction gas which was supplied at 200 cc/min for 20 minutes. After
repeating this procedure 10 times, the reaction tank was cooled and
the EL phosphor particles were collected to give coated EL phosphor
particles K. The EL phosphor particles thus collected had a
Ta.sub.2O.sub.5 coating layer having an average thickness of 100 nm
on the surface.
<Coated EL Phosphor Particles L>
[0141] Using the EL phosphor particles B, a coating layer made of
diamond carbon was formed on the surface of the particles by using
a rotated bed reactor as shown in FIG. 4. The rotated bed reactor
is an apparatus constructed by modifying a so-called rotary kiln. A
rotating quartz core tube 34 is located at an inclination of
1.degree. based on the horizontal direction. A microwave generator
35 is provided adjacent to almost the center of the quartz core
tube so as to allow microwave irradiation within the core tube. EL
phosphor particles 1 are supplied from the top edge of the inclined
tube with the use of a powder feeder 36. A coating layer material
gas and a back pressure gas 33 are supplied from a supplying tube
37 located at the end in the same side of the EL phosphor
particles. The other end is connected to a vacuum pump (not shown)
to evacuate the inside of the core tube. With the rotation of the
core tube, the EL phosphor particles supplied into the core tube
are slowly transported downward and packed in a powder-collecting
container 39 via a plasma generating area 38.
[0142] The EL phosphor particles were supplied into the core tube
by using the powder feeder and the core tube was rotated at 10 rpm.
While evacuating the inside of the core tube with the vacuum pump,
a gas mixture (CH.sub.4:H.sub.2=1:99) was supplied as the coating
material and the reaction gas so as to maintain the pressure in the
core tube at 5000 Pa. By irradiating with a microwave (2.45 GHz) at
300 W generated from the microwave generator, plasma state was
achieved in the inner space of the core tube and thus diamond
carbon was formed on the surface of the EL phosphor particles.
Thus, coated EL phosphor particles L were obtained. The EL phosphor
particles thus collected had a diamond carbon coating layer having
an average thickness of 50 nm on the surface.
<Coated EL Phosphor Particles M>
[0143] Using the EL phosphor particles B, a coating layer made of
Mg.sub.3 (Po.sub.4).sub.2 was formed on the surface of the
particles by using a liquid phase reactor as shown in FIG. 5. The
liquid phase reactor has a cylindrical solution-packing part 42
having a semispherical bottom in which a mother reaction liquor is
to be stored, an agitator 43 and at least one solution-supplying
pipe 44. The agitating blades of the agitator, which are in the
screw and paddle complex type, rotate so as to form an upward
agitation flow. A strainer 45 is located around the agitator. The
solution-supplying pipe is provided so that a solution is supplied
to the bottom of the strainer thereby. The solution-supplying pipe
is connected to a syringe pump 46 to thereby supply a reaction
liquor 41. The solution-packing part is heated/cooled with a water
jacket 47.
[0144] 2.5 L of distilled water and 12.2 g of
(NH.sub.4).sub.3PO.sub.4.3H.sub.2O were poured into the
solution-packing part and dissolved. To the obtained aqueous
solution, 100 g of the EL phosphor particles B were added and
suspended to give a mother reaction liquor. This mother reaction
liquor was heated to 40.degree. C. and agitated at 500 rpm. As a
reaction solution, an aqueous solution was prepared by dissolving
18.3 g of MgCl.sub.2.6H.sub.2O in 100 ml of distilled water and
packed in the syringe pump. By using the syringe pump, the reaction
solution was added at a speed of 2 ml/min. After the completion of
the addition of the reaction solution, the temperature of the
suspension was elevated to 90.degree. C. and the suspension was
matured over 1 hour. When the maturation was completed, the
suspension was cooled to room temperature and filtered under
suction though a 5C filter paper to thereby separate the coated
phosphor particles from the suspension. To the coated phosphor
particles remaining on the funnel in the form of a cake, distilled
water was added into the funnel thrice in 1 L portions and the cake
was washed by filtering under suction. After filtering, the cake of
the coated phosphor particles was vacuum dried by using a vacuum
dryer at 120.degree. C. for 4 hours. The coated phosphor particles
thus dried were annealed in the atmosphere at 300.degree. C. for 1
hour to give coated EL phosphor particles M. The coated EL phosphor
particles had an Mg.sub.3(PO.sub.4).sub.2 coating layer having an
average thickness of 200 nm on the surface.
<Coated EL Phosphor Particles N>
[0145] Using the EL phosphor particles B, a coating layer made of
MgF.sub.2 was formed on the surface of the particles by using the
same liquid phase reactor as used in the coated EL phosphor
particles M.
[0146] 2.5 L of IPA and 7.0 g of Mg(CH.sub.3COO).sub.2.4H.sub.2O
were poured into the solution-packing part and dissolved. To the
obtained aqueous solution, 100 g of the EL phosphor particles B
were added and suspended to give a mother reaction liquor. This
mother reaction liquor was heated to 40.degree. C. and agitated at
500 rpm. As a reaction solution, an aqueous solution was prepared
by dissolving 12.5 ml of CF.sub.3COOH in 87.5 ml of IPA and packed
in the syringe pump. By using the syringe pump, the reaction
solution was added at a speed of 2 ml/min. After the completion of
the addition of the reaction solution, the suspension was matured
over 2 hours. When the maturation was completed, the suspension was
cooled to room temperature and filtered under suction though a 5C
filter paper to thereby separate the coated phosphor particles from
the suspension. To the coated phosphor particles remaining on the
funnel in the form of a cake, distilled water was added into the
funnel thrice in 1 L portions and the cake was washed by filtering
under suction. After filtering, the cake of the coated phosphor
particles was vacuum dried by using a vacuum dryer at 120.degree.
C. for 4 hours. The coated phosphor particles thus dried were
annealed in the atmosphere at 300.degree. C. for 1 hour to give
coated EL phosphor particles N. The coated EL phosphor particles
had an MgF.sub.2 coating layer having an average thickness of 50
n=on the surface.
<Coated EL Phosphor Particle O>
[0147] Using the EL phosphor particle B, a coating layer made of
ethylene tetrafluoride was formed on the surface of the particles
by using a compound particle-constructing apparatus (a seater
composer) as shown in FIG. 6. The compound particle-constructing
apparatus has a rotor 49 having a large elliptic inner space
combined with a small elliptic rotor 50 having a major axis
somewhat shorter than the minor axis of the former rotor 49. These
large and small rotors are concentrically located and rotate in the
opposite directions to each other. A mixture of the EL phosphor
particles and the coating material 48 is poured into the space 51
formed by the large and small rotors.
[0148] 20 g of the EL phosphor particles B and 0.4 g of ethylene
tetrafluoride particles having an average particle size of 2 .mu.m
(TFW-3000F manufactured by SEISHIN ENTERPRISE CO., LTD.) were
poured into the seater composer. After rotating each rotor at 1000
rpm for 5 minutes, the EL phosphor particles were collected to give
coated EL phosphor particles O. The coated EL phosphor particles
had an ethylene tetrafluoride coating layer having an average
thickness of 200 nm on the surface.
[Evaluation of Coated EL Phosphor Particles]
[0149] Thickness of coating layer (measuring from SEM sectional
photographs).
[0150] Barrier properties (dipping in a 0.1 M AgNO.sub.3 solution,
observing a change in the body color and evaluating in 2 grades,
i.e., A (no change after 24 hours) and B (darkened))
[Fabrication of EL Devices]
[0151] EL devices were fabricated by using the coated EL phosphor
particles A to O obtained above and the EL phosphor particles A to
G having no coating layer.
[0152] A transparent electrode film I having an ITO electrode with
a surface resistivity of 100.OMEGA./.quadrature. layered on a PET
support (100 .mu.m) was prepared. Next, a transparent electrode
film II having an intermediate layer applied on the ITO electrode
surface was prepared. In the transparent electrode film II, a layer
of 1.5 .mu.m in thickness was formed by dissolving polyester of
bisphenol A and phthalic acid (terephthalic acid:isophthalic
acid=1:1) (U-100: manufactured by UNITIKA) in dichloromethane and
applying the solution (concentration 14%) by the dip-coating
method.
[0153] Next, each EL phosphor, a Cyanoresin (CR-S: manufactured by
SHIN-ETSU) employed as a binder and DMF as a solvent for dissolving
the binder were prepared. The following composition was added to
the organic solvent DMF and dispersed with a propeller mixer
(rotational speed 3000 rpm) to give a coating solution containing
the EL phosphor particles having a viscosity of 0.5 Pas at
16.degree. C. TABLE-US-00006 EL phosphor 100 parts by mass
Cyanoresin 25 parts by mass
[0154] The viscosity of each coating solution was measured by using
viscometers (VISCONIC ELD.R and VISCOMETER CONTROLLER E-200 ROTOR
No. 71: manufactured by Tokyo Keiki Co., Ltd.) under stirring
(rotation speed: 20 rpm) at a liquid temperature of 16.degree.
C.
[0155] Next, a solution (concentration 35% by mass) was prepared by
dissolving barium titanate (BT-8, average particle size 120 run:
manufactured by Cabot Speciality Chemicals) employed as dielectric
particles and a Cyanoresin (an equivalent mixture of CR-S with
CR-V: manufactured by SHIN-ETSU) employed as a binder in DMF. The
following composition was packed in a wide-mouthed bottle made of
Teflon and dispersed on a rotational roller at 50 rpm for 30
minutes, Then, 280 parts by mass of zirconia particles having an
average particle size of 2 mm were added thereto and the resultant
mixture was dispersed for additional 30 minutes.
[0156] The obtained dispersion was dispersed in a mix rotor
(consisting of parallel multiple discs made of alumina) for 2
hours. The rotational speed was 500 rpm at the initiation and then
gradually elevated to 2000 rpm as the dispersion proceeded. To
prevent the evaporation of the solvent, the dispersion was
maintained at about 20.degree. C. by ice-cooling around the pot.
After dispersing, 120 parts by mass of a 35% by mass Cyanoresin
solution and 54 parts by mass of DMF were added to the dispersion
and the mixture was dispersed for additional 20 minutes. The
obtained dispersion was filtered through a nylon mesh of 50 .mu.m
in pore size and defoamed. The filtered dispersion was packed in a
wide-mouthed bottle made of Teflon and dispersed on a rotational
roller at 50 rpm for 24 hours. Then, an appropriate amount of DMF
was added to give a dielectric particle dispersion having a
viscosity of 0.5 Pas at 16.degree. C. Immediately before coating,
this dielectric particle dispersion was passed through a 0.66 .mu.m
filter (manufactured by ROKITECHNO). TABLE-US-00007 BT-8 280 parts
by mass Cyanoresin 80 parts by mass DMF 25 parts by mass
[0157] Next, the dielectric particle dispersion was applied to an
aluminum base (thickness 80 .mu.m, unevenness in thickness .+-.3
.mu.m) by using a doctor blade coater provided with a bullnose
knife with a clearance adjusted so as to give a dry thickness of
20. .mu.m at a coating speed of 0.9 m/min. Next, it was dried in a
drying unit in which the temperature was gradually elevated from
110 to 130.degree. C. Thus, a dielectric layer was formed on the
back electrode.
[0158] Next, a solution (concentration 30% by mass) was prepared by
dissolving barium titanate (BT-8, average particle size 120 nm:
manufactured by Cabot Speciality Chemicals) employed as dielectric
particles and a Cyanoresin (an equivalent mixture of CR-S with
CR-V: manufactured by SHIN-ETSU) employed as a binder in DMF and a
red pigment having a light emission peak at a wavelength of 620 nm
was also prepared. The following composition was packed in a
wide-mouthed bottle made of Teflon and dispersed on a rotational
roller at 50 rpm for 30 minutes. Then, 280 parts by mass of
zirconia particles having an average particle size of 2 mm were
added thereto and the resultant mixture was dispersed for
additional 30 minutes.
[0159] The obtained dispersion was dispersed in a mix rotor
(consisting of parallel multiple discs made of alumina) for 2
hours. The rotational speed was 500 rpm at the initiation and then
gradually elevated to 2000 rpm as the dispersion proceeded. To
prevent the evaporation of the solvent, the dispersion was
maintained at about 20.degree. C. by ice-cooling around the pot.
After dispersing, 120 parts by mass of a 30% by mass Cyanoresin
solution and 54 parts by mass of DMF were added to the dispersion
and the mixture was dispersed for additional 20 minutes. The
obtained dispersion was filtered through a nylon mesh of 50 .mu.m
in pore size and defoamed. The filtered dispersion was packed in a
wide-mouthed bottle made of Teflon and dispersed on a rotational
roller at 50 rpm for 24 hours. Then, an appropriate amount of DMF
was added to give a dielectric particle dispersion having a
viscosity of 0.5 Pas at 16.degree. C. Immediately before coating,
this dielectric particle dispersion was passed through a 0.66 .mu.m
filter (manufactured by ROKITECHNO). TABLE-US-00008 BT-8 280 parts
by mass Cyanoresin 80 parts by mass Red pigment 17 parts by mass
DMF 25 parts by mass
[0160] Next, the dielectric particle dispersion was applied to an
aluminum base (thickness 80 .mu.m, unevenness in thickness .+-.3
.mu.m) by using a doctor blade coater provided with a bullnose
knife with a clearance adjusted so as to give a dry thickness of 10
.mu.m at a coating speed of 0.9 m/min. Next, it was dried in a
drying unit at 110.degree. C. Thus, a pigment layer was formed on
the back electrode.
[0161] To the dried dielectric layer, the coating solution
containing the EL phosphor particles was applied with a doctor
blade coater to give a dry thickness of 50 .mu.m and dried at
120.degree. C. Thus, a laminate composed of the back electrode, the
dielectric layer and the phosphor layer was obtained. In Example
16, however, the phosphor layer was applied in such a manner as to
give a dry thickness of 30 .mu.m.
[0162] The phosphor layer of each laminate thus obtained was heat
compression bonded to the transparent electrode film I or II with
the use of a laminator at 190.degree. C. The obtained laminate was
cut into pieces in A4 size and leader electrodes were attached to
the transparent electrode and the back electrode respectively. The
whole laminate was sealed with a moisture-proof film to give an EL
device. FIG. 7 shows the constitution of an EL device with the use
of the transparent electrode film II.
[Evaluation of EL Devices]
[0163] An AC voltage of 1 kHz was applied on each EL device and the
initial luminous efficiency and the brightness half-life in driving
at the voltage so that the initial brightness was 300 cd/m.sup.2
were measured. Table 2 shows the results. The brightness of the EL
device was measured with a brightness meter (BMp: manufactured by
TOPCON). The luminous efficiency was calculated by measuring the
power consumption in driving the EL device with a Power Multimeter
(7271; manufactured by NF KAIRO). TABLE-US-00009 TABLE 2 Light-
Light- emission emission Half- Half- efficiency efficiency EL
phosphor Phosphor life of life of of core of coated ContiNuity
Transparent Layer core coated particles particles Driving Test
Coated Core In electrode thickness particle particle (K.sub.0)
(K.sub.1) voltage No. particles particles coating film (.mu.m)
(H.sub.0) (h) (H.sub.1) (h) (lm/W) (lm/W) K.sub.1/K.sub.0 (V) Ex. 1
B (B) A I 50 750 1060 18.1 14.9 0.83 100 1-1 B1 (B1) A I 50 590 850
18.6 15.5 0.83 90 1-2 B2 (B2) A I 50 1000 1400 18.3 15.2 0.83 85 2
C (C) A I 50 670 920 16.7 13.5 0.81 100 3 D (D) A I 50 650 920 16.9
13.5 0.80 105 4 E (E) A I 50 570 860 16.0 12.5 0.78 100 5 F (F) A I
50 620 880 15.1 12.8 0.85 105 6 H (B) A I 50 740 1000 18.1 15.0
0.83 100 7 I (B) A I 50 740 1100 18.1 15.7 0.87 105 8 J (B) A I 50
740 880 18.1 13.2 0.73 100 9 K (B) B I 50 740 920 18.1 13.6 0.75 95
10 L (B) A I 50 740 950 18.1 11.8 0.65 100 11 M (B) B I 50 740 840
18.1 15.0 0.83 95 12 N (B) B I 50 740 870 18.1 15.6 0.86 95 13 O
(B) B I 50 740 810 18.1 13.4 0.74 100 14 B (B) A II 50 740 1200
18.1 14.8 0.82 105 15 K (B) B II 50 740 1250 18.1 13.0 0.72 100 16
B (B) A I 30 670 950 15.8 12.9 0.83 90 17 B' (B') A I 50 520 730
17.5 14.5 0.83 100 18 B'' (B'') A I 50 640 770 10.3 8.6 0.83 120 19
b (B) B I 50 750 760 18.1 15.2 0.84 100 C. A (A) A I 50 490 700 8.8
5.4 0.61 125 Ex. 1 C. G (G) A I 50 240 300 4.5 2.7 0.60 140 Ex.
2
[0164] As described above, it can be understood that the EL devices
using the coated EL phosphor particles of the invention (Examples 1
to 19) showed high luminous efficiencies and remarkably lowered
luminous efficiency changes (K.sub.1/K.sub.0) due to the coating
layer formation compared with the Comparative Examples 1 and 2.
[0165] These results indicate that the relative luminous
efficiencies were remarkably elevated in the embodiment of the
invention. It is also understood that the above effect became
remarkable by selecting an appropriate particle size, coating the
phosphor particles, gold-doping and platinum-doping. It is
furthermore understood that the above effect could be further
improved by selecting such a phosphor layer thickness as elevating
the luminous efficiency.
[0166] Compared with the comparative samples, the EL devices of
Examples of the invention required less voltage for light-emission
at the same brightness, indicating that a high luminous efficiency
and a high brightness were both established.
[0167] In the EL devices using the coated EL phosphor particles of
the invention (Examples 1 to 19), the brightness half-life could be
prolonged compared with the cases using uncoated EL phosphors
(comparison of H.sub.0 and H.sub.1), thereby achieving practically
efficacious lives.
[0168] From the results of Examples 14 and 15, it can be also
understood that the effect of prolonging the brightness half-life
could be synergistically elevated by introducing an intermediate
layer. By comparing the results of Examples 1 and 14 and the
results of Examples 9 and 15, it can be understood that a higher
synergistic effect could be achieved by using a coating layer
having less sufficient barrier properties. Furthermore, blackening
in the EL device was lessened, compared with the uncoated EL
phosphors, by combining the intermediate layer with the coated EL
phosphor particles, though this phenomenon is not shown by the
numerical data of the brightness half-life. In the case of a
coating layer having a poor continuity, the ion-barrier properties
are to be improved and the combined use with an intermediate layer
is effective therefor.
[0169] Moreover, the results of Examples 17 to 19 indicate that the
coated particles were highly durable in the case where the Au
content and the average thickness of the coating layer were within
the appropriate scopes as defined in the invention.
[0170] The EL devices of the invention had smaller particles and
smaller coefficients of variation in the particle size distribution
compared with the EL device of Comparative Example 2 and,
therefore, showed very low coarseness (granularity) originating in
the structural mottle.
[0171] This application is based on Japanese Patent application JP
2005-53415, filed Feb. 28, 2005, the entire content of which is
hereby incorporated by reference, the same as if set forth at
length.
* * * * *